This project seeks to prospectively examine the role of physical and psychosocial job factors in the development of occupational low back pain (LBP) during five successive phases: (1) the pre- disability symptom phase, (2) the pre-disability formal injury report phase, and (3) three disability phases - acute, subacute, and chronic - defined by increasing durations of lost work time. It is hypothesized that both physical and psychosocial job factors are independent predictors of LBP at all five phases and that their relative effect sizes change across phases. The main objective is to determine phase-specific risk factor profiles with particular focus on the relative impact of psychosocial and biomechanical risk factors. The long-term goal is to yield useful information for the design of workplace interventions which combine organizational and ergonomic job redesign to prevent low back injuries and work disability. This project combines the resources of two existing data sets - two prospective cohorts of San Francisco urban transit operators (n = 1,449, n = 1,640). These two longitudinal studies provide workers' compensation data with 3 to 5 years of follow-up, allowing for the study of each phase of the disability process, including the chronic disability phase (greater than 90 days off work), which accounts for 80 percent of the costs associated with work-related low back injuries. Both studies provide comparable information on job-related, sociodemographic, injury, medico- legal, and economic factors. Primary analyses will examine the independent and combined effects of physical and psychosocial job factors, including psychological and physical job demands, job control, job strain and social support at work. In addition, unique data on job stress, measured by independent observers with an innovative job analysis instrument, will be used for agreement analyses with self-reported measures to evaluate the predictive validity of survey instruments used in large epidemiological studies of occupational LBP.